CA1069489A - Catalytically active compositions from precious metal complexes - Google Patents

Abstract

ABSTRACT

An electroless plating catalyst prepared by reacting a tin salt in the molten state with a precious metal complex. The final catalyst may be a solid or a liquid at room temperature, depending upon the reaction conditions, and it can be reconstituted by dilution with an appropriate aqueous solution to prepare or replenish catalytic working baths for electroless plating. Examples include the molten salt reaction of precious metal complexes with anhydrous stannous chloride or stannous chloride dihydrate.

Description

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The present invention relates to cataly~ically active compositions for rendering the surface of a non-conductive substrate xeceptive to an electroless plating solution to form a uniformly adherent layer of metal. This layer, sometimes referred to as the preplate, may then be electrolytically plated in any conventional manner. It i5 :~
well understood in the art that the sensitizing step, des-cribed above, is preceeded by a surface treatmen-t which renders the substrate surface capable of forming a tight bond. This is normally done by etching in a strong oxidizing acid solution such as chromic acid, or a mixture of chromic and sul~uric acids.
U.S. Patent 3,011,920 (Shipley) describes a process in which a colloidal solution is prepared by mixing an agueous acid solution o^f palladium chloride with an aqueous acid solution of stannous chloride and optionally including a tin salt sucl ~s sodium stann~te. This is purported to produce a lyophilic colloid which, after acceleration with an acid or alkaline solution such as hydrochloric acid or sodium hydroxide provides a sensitizing layer for the sub-sequent electroless plating of a metal such as copper.
U.S. Patent 3,672,923 (Zeblisky) describes solid ~ compositions dilutable to optically clear sensitizing : solutions for electroless plating. These solutions are prepared by combining a dilute solution of a noble metal ; salt in hydrochloric acid with a hydrochloric acid solution of a stannous salt such as stannous chloride dihydrate.
The mixture is heated and ~hen subsequently cooled and evaporated to dryness under vacuum to constant weight.
The solid composition, as described, may then be recon- - :
~ stituted in hydrochloric acid to provide an active sensi- ~
, tizing solution. ~
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As described above in connection with the discu~ion of the prior art, the sol~tions heretofore recognized as being effective for catalytic sensitization of the surface are so-called palladium-tin systems in which a palladium salt, such as palladium chloride, and a tin salt such as s~annous chloride, are prepared by carefully mixing solutions (in aqueous hydrochloric acid) to form a solution which may or may not be colloidal in nature. It should be noted that whereas the Shipley patent purports to describe a colloidal system, the Zèblisky patent describes optically clear solutions which are stated to b~ noncolloidal in nature. In an~ event, the solutions in both cases are prepared by a re~ction in aqueous acid solution to form the sensitizing composition '-whether it be a colloid or a complex. Some problems may be ~-expexienced în preparing t~e Zeblisky dry catalyst compositions because of the diffic ~ in removing all excess water and hydrochloric acid. It is necessary to evaporate the solution to dryness to produce the solid compositions therein described, and the catalytic activity and stability can be seriously affected if water and/or acid remains after evaporation~ -It would, of course, be desirable to provide compositions in solid form because of their ease in handlin~.
This is especially true when considering the difficulty of replenishing an existing working bath. I~ the replenisher solution is added in relatively dilute li~uid form, it is normal practice to remove an equivalent volume of the exhaus~ed bath to make room for the addition~ If the materials can be added in solid form,it is only necessary to calculate the amount of composition needed to bring the bath up to ' '~
working strength and then add the solid catalyst. Moreove~r r ~, `' ~
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it iS obvious that shipping and storage of a dry material ~or liquid concentrate) would he more economical; and the fact that acid solutions are not involved reduces the safety hazards in handling the ca~alyst.
There are some practical limitations on how concentrated one can make known cataly~t solutions ~ithout running into crystallization and stability problems. The maximum concen-tration normal in commercial use is about four pounds of stannous chloride and 20 g of palladium chloride per gallon of sGlution. A solid catalyst, of the type described herein, can be made substantially of only stannous chloride and the catalytic palladium chloride - stannous chloridè reaction produce9 leading to much more concentrated and stable compositions.
The compositions herein described are prepared by reacting a molten tin halide component, such as stannous chloride and/or s~annous chloride dihydrate (with or without stannic chlorlde or its hydrates), with a noble metal saltO In order to stabilize the catalytic component, the present invention pro-posesto react a tin salt in the molten state with a precious metal complex which includes an appropriate complexing agent or chelating agent capable of in~eracting wi~h the precious metal used. ~
In one particular aspect the present invention provides a catalystically ac~ive composition suitable for initiating electroless deposition of metal comprising the reaction produce of a molten tin salt, a palladium salt, an effective amount of a metal halide compatible with said tin and palladium salts, and a metal complexing agent capable of forming a palladium complex.
~ ~ In order to bese understand the principles of the present inventlon, the following examples are provided for i~llustrative purposes only.

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Example I
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A stock solution of a palladium double salt, potassium chloropalladite (K2PdCl~ 9 in excess KCl solution, referred to herein as the "double sal~ mixture", was prepared . .
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by mixing 6.00 gms. of PdCl~ in 25 mls. of water and adding suficient KCl until the PdCl 2 dissolved. This required 6.51 gms. of KCl. The solution was diluted to lO0 mls.
volumetrically. To 8 n 3 mls. of the double salt mixture, 2.10 gms. of disodium EDTA were added based on a molar ratio.
of 2:1 for complexer to palladium ~II). After addition of the complexer, water was added to a volume of 20 mls. and the mixture was stirred 30 minutes~ The mixture was then evaporated to dryness. Fort.~ gms. of molten SnCl2-2H~0 :
were added and reacted at 95C ~or 20 minutes. An 18.1 gm. sample was dissolved in 243 mls. of reagent HCl (37~3 and 750 mls. of H20 to prepare a.catalyst solution for plating an ABS resin (Borg-Warner Chemicals EP-3510).
Standard test plaques were sequenced through a preplate cycle which included the following steps: (l) etching of the plaque in a chromic-sulfuric acid bathi (2) rinsing in water; (3) neutraliæing any remaining acid upon the surface; (4) sen~itizihg in the catalytic solution as des~
c~ibed above; (5) acceleration of the sensitizer, and (6) ..
immersion in an electroless nickel ba~h. A more detailed .:
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description of the pre~err~d concentrations and immersion ~. :.
times is found in "Pxeplate Systems" by John Ro~ertson, Products Finishing, VoL. 37, No. 4 (January 1973).
Coverage of the electroless nickel over the surface . .
of the plaques was good to excellent depending upon the pre~
plate procedure.
Example II ..
To 8.3 mls. of the double salt mixture stock solu~ion, . - -0.42 gms~ of glycine were added based on a molar ratio of 2:1 for complexer to palladium ~II). After addition of the . ~:;
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complexer, water was added to a volume of 20 mls~, stirred until dissolved, and evaporated to dryness. Forty gms. of molten SnCl2 2H 20 were added and reacted at 95C for 20 minutes. A sample weighing 17.3 gms. was dissolved in 243 mls. of reagent HCl and 750 mls. of H 20 to prepare a catalyst working hath. Coverage on A~S plaques processed th~ough this catalyst working bath was good to excellent depending upon the preplate procedure.
Example III
To 8.3 mls. of the double salt mixture stock solution, 1.23 gms~ o sodium gluconate were added based on a molar ratio of 2:1 for complexer to palladium (II). After addition of the complexer, water was added to a volume of 20 mls., stirred until dissolv~d, and evaporated to dryne~s. Forty gms. o~ molten SnCl2-2H20-were added and reacted at 95C ;
for 20 minutes. Twenty gms. were dissolved in 243 mls. o~
reagent HCl and 750 mls. of H20 to prepare a catalyst bath.
Coverage results on ABS plaques were excellent.
Example IV
The two-step addi~ion of SNCl2 2H20 to a noble metal complex was extremely effective for increasing catalytic activity D ' O~e gm. of Pd~12 and 3.36 gms. of KCl were dissolved in approximately 15 mls. of H20. A solution containing 4.20 gms. of disodium EDTA was added to the double salt mixture, .. ..
stirred, and evaporated to dryness. To this dried metal complex salt, 12.73 gms. of SnCl2 2H20 were added and reacted 15 minutes at 95C. Then 58.67 gms. of SnCl2~2H20 were added and reacted another 15 minutes at 95C. Nine gms. of the .
resultant product were dissolved in 121 mls. of reagent HCl and 375 mls. of H2Q to provide a catalyst working bath.

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Electxoless coverage of ABS plaques processed through this catalyst solution was excellent.
Example V
The presence of the KCl or other halide salt in combination with the noble metal complex in the molten salt reaction is very signi~icant as can be demonstrated by the ollowing experiment. One gm of PdClz was dissolved in approximately 10 mls. of reagent NH~OH. A solution of 4.20 gms. of disodium EDTA in 10 mls. of NH4QH was added to the palladium salt solution. The solution of Pd-EDTA
complex was evaporated to dryness. To the dried salt, 12.73 gms. of SnCl202H20 were added and reacted at 95C
for 15 minutes. Then 58.67 gms. of SnCl2 2H20 were add~d and reac~ed another 15 minutes at 95C. Nine ~ms~ of this material were dlssolved in 500 mls. of 3N HCl to produce a working bath. This working bath was not catalytic for elec~roless plating, , Example Vl ~ Another catalyst was prepared using PdSo4 in an acid medium~ Six mls. of a 10% Pd solution as PdSO4 in .
lON H2SO2 were added to 4.20 gms. of disodium EDTA in a weakly ammoniacal solution. This solution was evaporated to a substantially dry salt which was hlyhly acidic due to unneutralized H~SO2, and reacted with 71.40 gms. of molten SnCl2-2H~O at 95C for 15 minutes~ Nine gms. of thls catalyst were dissolved in 500 mls. of 3N HCl to prepare at catalyst ~ i working bath. Electroless coverage of ABS plaques processed thro~yh this catalyst was excellent.

From these examples it can be seen that, in order to prepare an effective catalyst using a palladium complex . ' ' ' ~ ' ~ " , , ph~

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with a molten sal~ synthesis, the following are necessary:
1~ an ef~ective complexer for palladium; 2) either an aci~
or an excess of halide ion~ or both.
Thus an active molten salt catalyst can be prepared from Pd - EDTA with excess halide ions, or from Pd-EDTA with free acid, or from Pd-EDTA with both excess halide and free acid. If, for example, a Pd-EDTA complex is used withou~
at least one of the two extra ingredien-ts, the desi.red catalyst is not formed.

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Claims (13)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A catalytically active composition suitable for initiating electroless deposition of metal comprising the reaction product of a molten tin salt, a palladium salt, an effective amount of a metal halide compatible with said tin and palladium salts, and a metal complexing agent capable of forming a palladium complex.

2. A composition as defined in Claim 1 wherein said tin salt is selected from the group consisting of stannous chloride dihydrate and anhydrous stannous chloride.

3. A composition as defined in Claim 2 wherein said tin salt is stannous chloride dihydrate.

4. A composition as defined in Claim 1 wherein said metal halide is an alkali metal halide.

5. A composition as defined in Claim 1 wherein said metal halide is an alkaline earth metal halide.

6. A composition as defined in Claim 4 wherein said alkali metal halide is potassium chloride.

7. A composition as defined in Claim 1 wherein said metal halide is selected from a group consisting of chloride and bromide.

8. A composition as defined in Claim 1 wherein said palladium salt is palladium chloride.

9. A composition as defined in Claim 1 wherein said complexing agent is a negative or positive ion or polar molecule bound to the palladium by a metal-ligand bond.

10. A composition as defined in Claim 9 wherein said complexing agent is a chelating agent.

11. A composition as defined in Claim 10 wherein said chelating agent is EDTA or salts thereof.

12. A composition as defined in Claim 10 wherein said complexing agent is glycine.

13. A composition as defined in Claim 9 wherein said complexing agent is gluconic acid or salts thereof.